Whether it's served in a demitasse mug or a venti mochachino bucket, coffee is an essential, eye-opening morning ritual for many of us. But at what point does throwing back another vente doing you more harm than good?

As you probably know, coffee's main health concern—its caffeine content—is also why you drink so much of it. So what exactly is caffeine doing to your body, and how much of it can you safely handle?

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What is caffeine?

Caffeine is a naturally-occurring crystalline alkaloid derived from the leaves, seeds, and fruit of a number plants including the coffee tree and tea bush, as well as less common yerba maté and guarana berries. Plants produce the compound as both a natural insecticide against harmful bugs and as a reward for pollinators.

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In humans, however, it acts as a neurostimulant and mild psychoactive, invoking a temporary state of alertness and focus. In fact, caffeine is the world's most widely consumed psychoactive drug, and can be found in everything from coffee and tea to energy drinks and chocolate. Altogether, some 90 percent of adults in North America consume caffeine daily in one form or another.

"It helps people stay alert and stay focused," Lisa Casas, R.D., L.D., a clinical dietitian at University of Iowa Hospitals and Clinics, told the University's blog. However, caffeine's boost is transient at best. "It really doesn't stay in the system very long—it's only about two to three hours before it is excreted from the body," Casas continued.

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How much caffeine is in coffee?

Coffee can contain anywhere from 60 to 120 mg per 8-ounce serving, depending on how it is brewed. Teas contain far less, averaging 20 to 90 mg per 8-ounce cup, as do soft drinks, which pack around 20 to 40 mg per can. Even chocolate contains a small amount of caffeine, about 6 mg per ounce.

Energy drinks, on the other hand, contain far more caffeine than any of those potables. A standard 1.93 oz dose of 5 Hour Energy holds a whopping 207 mg, according to a 2010 Consumer Reports study. Similarly, Monster Energy drinks pack around 160 mg per 16 oz can, though the 24 oz MegaMonster variant contains 240 mg. Red Bull contains an equal proportion of caffeine as MegaMonsters (80 mg per 8 0z), though they're a quarter of the serving size.

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However, all of these drinks pale in comparison to the 5150 Semi Sweet energy shot. Each 1 ounce dose of 5150 delivers a heart-bursting 475 mg of caffeine—that's 2.6 times the amount of stimulant you'd get from downing an 8-ounce Starbucks brew.

While the study is alarming, it's also important to note that correlation is not causation. "High coffee consumption could just be associated with increased death risk," Carl J. Lavie, a cardiologist at the John Ochsner Heart and Vascular Institute in New Orleans and one of the study authors, told Men's Journal. "We were able to control for smoking and fitness, but not for sleep deprivation, stressed-out life, and other factors that could play a part."

"This result is surprising," Rob van Dam, Adjunct Associate Professor of Nutrition and Epidemiology at the Harvard School of Public Health, told PBS via email, "because results from other cohort studies in U.S. men and women suggest that coffee consumption is associated with a slightly lower risk of premature mortality."

The Mayo study appears to support another recently published study from the New England Journal of Medicine that found people with a daily coffee habit had a lower risk of dying during the 14-year study period than non-drinkers but that effect diminished the more cups participants drank daily.

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Since caffeine can affect people differently depending on their sex, age, body mass, metabolic rate, and sensitivity to the stimulant, there is no hard and fast limit on how much caffeine one can consume in a day. The highest survivable dose on record is a full 100 g. Not milligrams, grams—that's equivalent to 1,200 cups of coffee in a single sitting. However, it's safe to say that consuming anywhere near that amount will likely prove lethal for most people regardless of their tolerance.

The FDA recommends that people with high blood pressure, heart disease, and pregnant women keep their caffeine consumption to a minimum. If you're 55 or older, studies suggest keeping it to fewer than four cups a day. For the rest of us, recent studies suggest that a moderate intake—between 200 and 400 mg per day—generally doesn't cause long-term harm for the average healthy adult. More than 600 mg per day—equivalent to about seven cups of coffee—however, is pushing it no matter who you are.

If you experience any of the following symptoms, regardless of how many cups you've had, you'd best put down that coffee mug and back up off it: